Sandwich panels are used in a wide variety of applications requiring structural and/or thermal insulation properties. These applications include structural and non-structural uses in refrigerated and non-refrigerated buildings, boats, aircraft, rapid transit and recreational vehicles, enclosed trailers and many others. Structural sandwich panels are composite structures formed by bonding two generally thin facings or skins to a relatively thick core material. The skins, which are normally dense and strong, resist compression and tension, while the core, which is normally of relatively weak and low density material, serves to separate the skins, stabilize them against buckling and resist shear loads.
Two common materials used as cores for sandwich panels are rigid expanded plastic foams and honeycomb materials. Honeycomb core usually comprises a thin sheet material, such as paper or aluminum foil, which is formed into a variety of cellular configurations. Expanded plastic foam cores usually provide much higher levels of thermal insulation than honeycomb, but honeycomb cores are normally substantially stronger than insulating foam cores of comparable density.
Various methods of introducing insulating foams into the cells of honeycomb have been used for the purpose of filling the voids or adding higher levels of thermal insulation to structurally adequate honeycomb core. These include such approaches as applying foaming chemicals to the honeycomb cells, and pressing slabs of plastic foam into the cells. However, these processes are difficult to perform in thick core sections, limit the types of foams which can be used to fill the cells of the honeycomb uniformly, or require large capital investment in machinery. As a result, such composite cores have enjoyed little use in most sandwich panel applications, and many honeycomb core products are consequently deficient in insulation and subject to migration of water into the core.
Sandwich panels with skins of metal, wood, fiberglass reinforced plastics and similar durable materials are widely manufactured by three basic processes. In one process, liquid chemicals, commonly of polyisocyanurate formulation, are injected between the skins, after which they react and expand to form a rigid foam which bonds itself to the skins to form the sandwich panel. A second method of producing sandwich panels is by adhesive lamination wherein preformed panel skins are bonded by adhesive to cores of rigid foam boards or slabs which have been cut from expanded foam billets. In the third method, uncured resins and reinforcing materials are applied to the surfaces of such foam boards, or resins are introduced and into closed or vacuum bagged molds containing the core and skin reinforcements and subsequently cured to form rigid skins.
Sandwich panel laminators use a wide variety of these preformed cores, including polyurethane, polyisocyanurate, extruded polystyrene, expanded polystyrene, polyvinylchloride and foam glass. The use of common low density (2.0 pounds per cubic foot) polyurethane and polyisocyanurate foams in structural lamination processes is severely restricted in spite of large demand for sandwich panels containing such cores. The cost of such core materials in the form of boards or slabs cut from billets is substantially higher than the cost of chemicals used in foam-in-place processes, a differential of typically two to three times. In addition, since most polyurethane and polyisocyanurate billet stock is manufactured for applications other than sandwich panels, the stock is not usually formulated with physical properties designed primarily for structural sandwich panel use. The best foam stocks having appropriate properties for such sandwich panels are produced by very few suppliers and in limited geographic areas.
Polyisocyanurate foam is also produced as board stock with attached facings for use as insulation in roofs and other non-structural construction applications. This foam material satisfies common fire performance specifications, is manufactured in large quantities in numerous locations, and is sold as a relatively low priced commodity. In spite of its compelling cost advantages and the modest structural requirements of most construction panel cores, such insulation has seen very limited use as core material for sandwich panels. Both thickness and flatness of the roof insulation boards have unacceptably wide tolerances for most sandwich panel applications. While planing or sanding the facings eliminates this problem, it also removes the skin part of the foam board having the highest density and strength. Even more serious, available polyisocyanurate foam formulations are not consistently tough enough, and their friability, brittleness and low shear strength can result in serious structural failures. These show up as delamination or foam shear under conditions of structural loading, thermal stress or surface impact.
Plastic foam cores for more structurally demanding sandwich panel applications, such as the hulls of boats, are commonly made of linear or cross-linked polyvinyl chloride (PVC) formulations, in densities of from 2 to 16 pounds per cubic foot. The high cost of these materials per board foot has limited their use in such major medium to high performance applications as highway trailers and recreational vehicles. A further drawback of the PVC foams and of other thermoplastic foams, such as polystyrene, is serious degradation of their physical properties at elevated temperatures encountered in many transportation and other environments.
Plastic foam core sandwich panels often involve serious compromises in their design and cost due to inherent structural limitations of the rigid foam insulation cores. In addition to the deflection of these panels due to compressive and tensile stresses in the skins, further deflection results from the relatively low shear modulus of the rigid foam material. The thicker the core, the more important shear deflection becomes, to the point of exceeding deflection due to bending. Under a sustained load, the plastic foam core is also subject to creep deformation, further increasing panel deflection, with resulting risk of failure of the sandwich panel.
These deficiencies of the core may require increasing the strength and stiffness of the composite through the use of excessively heavy and expensive skins. Alternately, the panel could be improved structurally by increasing the thickness or density of the foam core beyond acceptable limits, which also raises the costs of both material and shipping. The relatively low shear modulus of low density plastic foam cores also allows buckling of thin flat panel skins to occur at relatively low stress levels, again calling for overdesign of skins or higher density foam cores as a compensation. Low shear resistance and the absence of reinforcing elements within the foam core also permit the propagation under stress of cracks or fissures between the core and the panel skins as well as within or through the core itself, with resulting deterioration or structural failure of the panel. Still another difficulty is the low compressive strength of most plastic foams, which allows concentrated or impact loads to distort both skins and core.
Reinforcing frames or ribs of metal, wood, fiberglass reinforced plastic and other materials have been used in foam core sandwich panels to mitigate or overcome the structural limitations described above. Although both foam core and ribs contribute to the strength of these panels, the structural contribution of the ribs in such constructions is not fundamentally dependent upon the presence of the foam core.
An often serious drawback of widely spaced ribs is the creation of overly rigid sections of the structure within a generally more flexible panel. This can result in undesirable concentrated loads at the intersection of ribs and face laminates, especially with thinner face laminates made with higher strength composite materials. Structural properties of the composite may be improved by assembling between the skins a large number of individual blocks or strips of foam wrapped with fibrous reinforcing materials which connect the skins and fill the space between them. Impregnating resins are applied to both skin and core reinforcements during this layup process. Alternately, all components of skin and core reinforcement and foam may be positioned in a mold while in a dry and porous state, after which the mold is closed and resin is introduced under pressure, as in vacuum-assisted resin transfer molding, to flow into and impregnate the reinforcements. In either case, the process of preparing and inserting the individual foam and reinforcement components of the core is both labor intensive and expensive.